One kind of monitoring camera adapted for use in both of the visible region (in the daytime) and the near-infrared region (at night) is of the type in which an infrared cutoff filter for cutting off near-infrared light is positioned on the optical axis in front of an imaging device (CCD) when the camera is used to take images in the visible region, while a dummy filter replaces the infrared cutoff filter when the camera is used to take images in the near-infrared region, so that it may be possible to correct any deviation of image focus location occurring under the influence of axial chromatic aberration.
The mechanism for an infrared cutoff filter as stated above (hereinafter referred to as the infrared cutoff filter mechanism) has hitherto been installed in a camera body with an imaging device. In a recently sold monitoring camera having a lens barrel forming an integral part of the camera body, however, the installation of the infrared cutoff filter mechanism in the camera body has been made difficult by a reduction in size of the camera body, etc.
Accordingly, there has been proposed a technique for installing an infrared cutoff filter mechanism in a lens barrel (see, for example, JP-A-2002-189238). According to the literature, an infrared cutoff filter mechanism is integrally connected to an aperture mechanism and installed in a lens barrel.
FIG. 3 is a schematic illustration of a monitoring camera equipped with a lens barrel having an aperture mechanism connected integrally to an infrared cutoff filter mechanism. As shown in FIG. 3, the monitoring camera 200 includes a camera body 202 and a lens barrel 204. The lens barrel 204 includes a front fixing cylinder 206 for fixing a first lens (group) 212, a rear fixing cylinder 208 for fixing a second lens (group) 214 and a CCD fixing cylinder 210 for fixing a CCD 216. An aperture mechanism 218 connected integrally to an infrared cutoff filter mechanism is positioned between the front and rear fixing cylinders 206 and 208.
In the aperture mechanism 218, an aperture 220 formed by two aperture blades and a filter fixing plate 222 for fixing an optical filter, such as an infrared cutoff filter, are disposed along the front and rear surfaces, respectively, of a base plate 224. The aperture mechanism 218 also includes on the outer periphery of the lens barrel 204 an aperture driving device 226 for adjusting the size of a clear aperture in the aperture 220 and a filter driving device 228 for installing and uninstalling the optical filter on the optical axis.
However, the presence of the aperture mechanism connected integrally to the infrared cutoff filter mechanism between the first and second lenses 212 and 214 as shown in FIG. 3 makes it necessary to design an especially wide spacing between the lenses as compared with any ordinary lens barrel. Moreover, the provision of the aperture driving device 226 and the filter driving device 228 on the outer periphery of the lens barrel 204 makes it necessary to design new component parts, etc. on the outer periphery of the lens barrel 204. Accordingly, they fail to be common with many component parts on any ordinary lens barrel, but objectionably add to the cost of manufacturing the lens barrel.
The integral connection of the aperture and infrared cutoff filter mechanisms makes it necessary to disassemble them as a whole in the event of failure of any component part of either of them to examine the cause of its failure and repair it.